In this paper, we design an exponentially convergent disturbance observer for a wave PDE on a time-varying domain by using two boundary measurements u(0, t), ut(l(t), t). More specifically, two auxiliary PDEs are constructed to build the disturbance observer for tracking the external disturbance in the wave PDE. Exponential convergence of the disturbance estimation to the true disturbance value is proved by Lyapunov analysis, and all states in the observer are shown to be bounded once the original state u(x, t) is bounded via designing control input.